Currently, flexible printed circuit boards mainly rely on flexible copper clad laminates (FCCL) which are classified into non-adhesive flexible copper clad laminates (2-layer FCCL) and adhesive flexible copper clad laminates (3-layer FCCL) by the number of layers, and the greatest difference between those two is whether there is an adhesive between a copper foil and a polyimide thin film. 2L FCCL has the advantages of high heat-durability, high warp resistance, and improved dimension stability. However its cost is comparatively higher. Therefore, most flexible substrates mainly use 3L FCCL, only higher level flexible substrates adopt 2L FCCL.
3L FCCL requires using epoxy resin as an adhesive between a flexible substrate and wires. However, the heat-durability of a general epoxy resin adhesive is worse than that of polyimide. Therefore, there is a temperature restriction on the use of an epoxy resin adhesive. Additionally, the reliability thereof is unsatisfactory. However, in order to meet the requirements of wire patterning, 2L FCCL also requires the steps of surface treatment, film deposition and etching whose manufacturing procedure is complicated and time-consuming.